1. Technical Field
This invention relates to the field of transmitting high definition television signals and, more particularly, to a method and apparatus for increasing the vertical definition of a transmitted television signal without increasing bandwidth.
2. Background Information
Television pictures are an assembly of horizontal lines, each line being modulated with an amplitude signal representing the luminance or chrominance component of the picture. These line signals are sequentially transmitted and received for display on a cathode-ray tube forming the original picture. Two separate line standards are predominant. In North America and Japan, television transmissions are based on 525 horizontal lines per picture. In Europe, Africa, Asia and Australia, television transmissions are based on 625 lines per picture. The higher the number of lines of an image, the higher its clarity or vertical resolution. In recent times, technology has become available to display many more television lines, which improves picture quality as it increases vertical definition. Components are also available which allow an increase in the bandwidth of the line signals which improves picture quality by increasing horizontal definition. Therefore, technology exists which allows for a much higher picture quality than is available from existing standard transmissions. This technology is known as High Definition Television (HDTV). In particular, methods have been proposed based on a display of 1050 lines, 1125 lines and 1250 horizontal lines. The main problem with these recent proposals is that there is no economical way to introduce such transmissions. The television receivers currently in use throughout the world will receive only 525 or 625-line transmissions and are incapable of displaying HDTV transmissions. The dominant cost of providing television service is the cost of programming which must be funded through, for example, advertising or pay-tv. Such methods of funding require the existence of a large audience. During any start-up phase of providing HDTV service, there will be no compatible receivers and therefore no possibility of recovering programming costs. Also the cost of an HDTV receiver is likely to be discouragingly high.
One way to avoid this difficulty is to introduce the concept of existing receiver compatible HDTV transmissions which can be converted to a signal of conventional format using a converter of low cost. Such a low-cost converter would allow display of the HDTV transmissions on a conventional receiver but achieve conventional picture quality. This approach encourages a more rapid growth of the viewing audience, since new subscribers to the service would not initially have to buy an expensive HDTV receiver but could acquire a low cost converter for their present television receiver. Only those subscribers who wish to achieve the higher picture quality available through HDTV would acquire the more expensive HDTV receiver.
Several approaches to the design of a compatible HDTV signal have been suggested, all based on an HDTV line standard which is a simple multiple of the existing line standard. For example, in North America where the existing transmission standard is 525 lines/picture, a suggested HDTV standard will employ 1050 lines (twice the conventional standard). Similarly in Europe, where the conventional standard is 625-lines/picture, a suggested HDTV standard is 1250 lines. This simple relationship between the conventional and HDTV display standards suggests transcoding between the two standards, for example, by discarding alternate lines or by re-interpolating missing lines. The present invention and its back-ground will be discussed throughout the present application in terms of the proposed 1050/525 lines standard applicable in North America, it being understood that the concepts apply equally to the 1250/625 line standards proposed in Europe. Small variations from the numbers indicated are possible while retaining the essential property of the present invention, where, in particular, alternate lines are decimated and reinterpolated during transcoding. Several design options have been proposed employing alternate methodology.
One suggested approach is to transmit a standard television signal and a supplemental signal. Thus, one component of the transmitted signal is directly compatible with conventional receivers without the requirement for any converter. The supplemental signal may be transmitted on a separate channel or multiplexed with the first signal such that existing receivers are not affected by it. HDTV receivers would receive both of these signals and use them to reconstruct a high-definition picture. In one such proposal, the supplemental signal consists of an analog signal carrying information relating to the difference between the 525 standard transmitted lines and the intervening 525 lines which must be reconstructed. Addition of the difference information to the transmitted compatible lines reproduces the missing lines. The supplemental signal may be transmitted in a second channel or on a subcarrier with the first signal.
The problem with these techniques is that they require absolute compatibility; that is, the first signal must be an essentially conventional composite National Television Subcommittee (NTSC) signal (or PAL/SECAM in Europe). These composite signals employ a color subcarrier which gives poor chrominance resolution. The color subcarrier is difficult to enhance for improved chrominance definition and introduces significant distortion to the luminance signal. With the NTSC standard, it is very difficult to adequately separate luminance and chrominance to form high quality signals as the basis for high-definition pictures.
On the other hand, absolute NTSC compatibility is probably not a stringent requirement. It is likely that any practical HDTV standard will include a capability for conditional-access (pay-tv) reception. Scrambled television signals require a transcoder for each receiver; therefore, any signal transmission format can be used, provided that the cost of transcoding to conventional NTSC is not unduly expensive in a subscriber decoder. The NTSC signal was standardized approximately 40 years ago and is now being overtaken by advancing technology. Since 1980, new composite signals (based on digital processing) have been developed which provide better picture quality than NTSC and which provide a better basis for HDTV. There are, for example, formats based on digital time compression for multiplexing luminance and chrominance components, i.e., Multiplexed Analog Components (or MAC) signals, also known as Time Multiplexed Components (TMC). When such signals are transmitted in 525-line format (e.g. 525-line MAC), transcoding to NTSC in a decoder may be achieved at low cost.
MAC signals do not employ a color subcarrier to carry the color information. Instead they employ time compression to multiplex luminance and color information within each transmitted television line. This technique is illustrated in FIG. 1. In the decoder, the luminance and chrominance components are stored and separately decompressed to give the full-line luminance and color signals required for display. This allows complete separation between the components and avoids the cross-interference effects associated with the NTSC color subcarrier. For this reason, most HDTV proposals have been based on time-compression techniques.
When the MAC signal employs a 525-line structure, it may be simply transcoded to a conventional NTSC signal in the decoder to maintain compatibility with existing receivers. This may be achieved using one or two low-cost custom integrated circuits.
The more encouraging proposals for HDTV start with a 525-line MAC/TMC signal which may be simply converted in a subscriber decoder to NTSC and at the origin of transmission to 1050-line component HDTV. The most important technical question that remains concerns the technique to be used to enhance the definition, particularly in the vertical direction, while maintaining the possibility of low-cost decoding to NTSC.
One further approach to compatibility would be to transmit all 1050 lines. Alternate lines could then be discarded at the subscriber decoder using a line memory store for the remaining lines, leaving a 525-line MAC signal ready for conversion to NTSC. HDTV receivers would display all 1050 lines.
The problem with this approach is the excessive bandwidth required for transmission of such a signal. For example, if it is required to double the horizontal bandwidth from 4 MHz (in NTSC) to 8 MHz (equivalent), and in addition to double the number of lines from 525 to 1050, the transmission bandwidth increases by a factor of 4. This is not an economic solution in terms of transmission costs due to the excessive demands on R. F. spectrum.
A modification of this approach has been suggested in which two separate 525-line signals are transmitted. A first MAC signal is available for direct conversion to NTSC, while an HDTV receiver decodes both signals to produce a 1050-line result. While this approach eliminates the need for memory stores in the NTSC-compatible decoder (transferring this cost to the HDTV receiver), it does not address the question of the excessive demand for R. F. spectrum.
Another approach which has been extensively investigated is the potential improvement in vertical definition achievable by employing interlace lines from previously transmitted fields to reinterpolate missing lines. Referring to FIG. 2, a 525-line interlace signal can be converted to a 525-line sequential signal (double the number of lines) by using lines from previous fields to reinterpolate missing lines. This technique has the potential to increase subjective vertical definition by approximately 50%. With appropriate sample structures, the use of information from previous fields has the potential to increase the definition in both the vertical and horizontal directions.
There are two problems with this approach and which apply to all techniques employing information from previous fields. The first is the consideration that the HDTV receiver will need to employ a field store (perhaps more than one) to reconstruct the entire HDTV image. Although field stores are not yet available at reasonable costs, it is predictable that manufacturing in sufficient volume will solve this problem in the forseeable future. The second objection is more severe. Interpolation of missing lines using information from previously transmitted fields is effective only for static picture elements. It is therefore necessary to employ more complex techniques for moving pictures.
Two approaches are being used experimentally to solve the problem:
(i) Detect moving elements of the scene, and (in areas of movement) employ intrafield interpolation (from surrounding lines of the same field).
(ii) Detect not only the fact of motion, but also its magnitude and direction, allowing interpolation from the appropriate segments of surrounding fields.
Neither of these approaches has yet provided a convincing demonstration that adequate representation of moving pictures can be achieved in a decoder that an HDTV subscriber could afford in the forseeable future.
One development of technique (i) above eliminates the need for motion detection in the consumer decoder. This method employs sophisticated equipment at the transmitter to detect motion in segments of the scene and signals this information to all decoders using a separate digital channel (digitally assisted television). This work is currently in its early stages; however, preliminary results suggest that a significant data capacity may be required. In order to be viable, data requirements associated with this approach must be reduced to acceptable levels without increasing the cost and complexity of HDTV decoders. Thus, it is clear that there remains a requirement for a viable alternative solution to the problem whereby bandwidth may be preserved, yet the HDTV subscriber, with either a conventional television receiver or a new HDTV terminal, may be economically served.